A. Field of the Invention
The invention relates to a damper mechanism and particularly to a damper mechanism for damping torsional vibrations in a power transmission system.
B. Description of the Background Art
A clutch disk assembly used in, for instance an automotive vehicle, is typically installed in a clutch mechanism such that the clutch disk assembly can be used in clutch engagement and clutch dis-engagement operations for transmitting torque from a flywheel to a transmission input shaft. The clutch disk assembly preferably also includes a vibration dampening function for absorbing and damping vibration transmitted from the flywheel. Generally, vibrations of a vehicle include idling noises (rattle), driving noises (acceleration/deceleration rattle and muffled noises) and tip-in/tip-out (low frequency vibrations). The clutch disk assembly has the above damper function for removing these noises and vibrations.
The idling noises are rattling noises which occur from a transmission when the transmission is in a neutral position, e.g., during waiting at traffic signals with clutch pedal off. This rattling occurs due to the fact that engine torque is low in an engine idling range and engine combustion causes large torque variations in the idling range. In this state, gear contact occurs between an input gear and a counter gear of a transmission, and thereby noises are produced.
The tip-in/tip-out low frequency vibrations are large longitudinal vibrations of a vehicle which occur when a driver rapidly depresses or releases an accelerator with the clutch in an engaged, torque transmitting condition. If rigidity of a drive transmission system is low, torque transmitted to wheels is transmitted or reflected from the wheels back through the drive train creating large oscillations of torque.
In a state where no torque is transmitted (zero torque transmission), for instance during idling, the dampening characteristics of most clutch disk assemblies are such that idling vibrations cannot be adequately dampened creating corresponding noises. Therefore, a low torsional rigidity is preferable in this region of zero torque transmission. Contrarily, it is necessary to maximize the rigidity of the torsion characteristics of the clutch disk assembly for suppressing the longitudinal vibrations of the tip-in/tip-out.
For overcoming the above problems, a clutch disk assembly which uses two kinds of springs for achieving vibration dampening characteristics in two separate stages has been provided. This structure has a low torsional rigidity and a low hysteresis torque in the first stage of a low torsion angle, and therefore can achieve an effect of preventing noises during idling. Since the torsional rigidity and the hysteresis torque are high in the second stage of a high torsion angle, the longitudinal vibrations at the time of tip-in/tip-out can be effectively damped.
A damper mechanism is already known where operation of a high hysteresis torque generating mechanism in a second stage with a high torsion angle is prevented when minute vibrations caused, e.g., by combustion variations of an engine are supplied in the second stage region. Thereby the minute vibrations are effectively absorbed by a low hysteresis torque.
In a damper mechanism of the conventional clutch disk assembly described above, torsional operation is repeated through a wide angular range in and between the positive second stage and the negative second stage in the torsion characteristic when low frequency vibrations are supplied thereto. Therefore, only a low hysteresis torque is generated in the region of the positive and negative first stages between the positive and negative second stages. Accordingly, all of the vibrations can be damped only to a small extent, and low frequency vibrations cannot be damped sufficiently. Further, the regions of the positive and negative first stages may form a gap or space in the torsion characteristics, resulting in an undesirable increase in longitudinal vibrations.